1. Field of the Invention
This invention relates to a process for the preparation of an indigo compound by reacting an indol compound having no substituent at the 2- and 3-positions with hydrogen peroxide in a specific liquid-phase reaction system.
2. Description of the Prior Art:
Indigo compounds are important compounds that are useful as dyes. The presently employed industrial processes for the preparation of indigo comprise forming an N-phenylglycine salt from aniline and chloroacetic acid, or from aniline, cyanic acid and formaldehyde, converting this salt into an indoxyl compound by alkali fusion at elevated temperature, and then oxidizing this compound with air. However, these processes are not only complicated ones having many steps, but also require the use of large amounts of potassium hydroxide and sodium hydroxide. Moreover the recovery and reuse of used potassium hydroxide and sodium hydroxide has the disadvantage of consuming much energy and requiring special equipment. Therefore, conversion to a simpler process has been desired.
Meanwhile, a report has been published in which indole was reacted with an aqueous solution of hydrogen peroxide in a liquid phase made homogeneous by the addition of methanol (Khim. Geterotsikl. Soedin., Vol. 11, pp. 1490-1496, 1978). It is stated therein that a trimer of indole, or 2,2-diindyl-.psi.-indoxyl, was formed in high yield and, in addition, indigo could be detected by chromatography. However, the object of this report was to prepare 2,2-diindyl-.psi.-indoxyl by the oxidation of indole, and the indigo that is the desired product in the present invention was nothing but a by-product formed in very small amounts. Accordingly, this is not a satisfactory process for the preparation of indigo compounds.